1. Field of the Invention
The present invention relates to the field of telecommunications and, more particularly, to a thermal protection circuit for a subscriber line interface that monitors the operating temperature of the subscriber line interface circuit and annuls power dissipation when a designated temperature is reached.
2. Description of the Prior Art
Telephone line circuits are customarily found in the telephone switching system or central office of a telecommunications network. The telephone line circuit interfaces the central office to a telephone or subscriber station found at a location remote from the central office. The telephone line circuit functions to supply power or battery feed to the subscriber station via a two wire transmission line or subscriber loop and to couple the intelligence or voice signal to and from the telephone switching system.
In many presently known telephone line circuits the battery feed function has been performed by using a passive, highly balanced, split winding transformer and or inductors which carry up to 120 ma dc. This passive circuit has a wide dynamic range, passing noise-free differential signals while not overloading with the 60 Hz longitudinal induced currents. The line circuit just described, feeds dc current to the subscriber loop and also provides the voice path for coupling the voice signal between the subscriber station and the central office. The electromagnetic components of passive line circuits are normally bulky and heavy and consume large amounts of power for short subscriber loop lengths where the current fed to the subscriber station is more than necessary for equalization. Active line-feed circuits can be less bulky and require lower total power, but meeting dynamic range and precision balance requirements dictates an overly complex circuit design.
Recently, solid state replacements for the electromagnetic components of the aforementioned line circuits have been developed. Devices such as high voltage bipolar transistors and other specialized integrated circuits are being designed to replace the heavy and bulky components of the electromagnetic line circuit. Such a device is described in the IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, Aug. 1981, entitled, "A High-Voltage IC for a Transformerless Trunk and Subscriber Line Interface." These smaller and lighter components allow the manufacture of telephone switching systems having more line circuits per circuit card as well as decreasing the physical size of the switching system.
The solid state line circuits just described are in many instances manufactured as an Integrated Circuit (IC). Such ICs are susceptible to destruction due to high applied heat or some other abnormal condition which allows the IC to heat-up beyond its normal operating limits. Whenever an IC is subject to high-temperatures, such as, on-chip power dissipation, high working ambient temperature, or due to some faulty condition, thermal protection arrangements such as current limit circuits are used to cut power to the circuit.
In many such ICs, thermal protection circuits are employed that sense temperature or current which is subsequently converted into a voltage. This voltage is then compared with a known voltage and a switch is provided to shut-down or limit the current and power dissipation of the IC.
One such circuit is taught by U.S. Pat. No. 4,553,048. This thermal protection circuit uses a band-gap voltage circuit to generate a reference voltage that is compared with the V.sub.BE of a transistor. It is well known by those
skilled in the art, that V.sub.BE has -ve temperature coefficient, while the reference voltage (V.sub.REF) generated using the band-gap technique has a +ve temperature coefficient. At a designed temperature V.sub.REF, will be higher than V.sub.BE causing a switch to turn-on, thereby shutting down the IC and limiting power dissipation.
The circuit just described has limitations in that, the generated V.sub.REF varies considerably due to process variation and current flow through the band-gap circuit. Further, once the temperature of IC falls back into the normal operating limit, even minor power dissipation may cause the protection circuit to shut-down the IC again, allowing the IC to go into on/off oscillation. The oscillations can inject noise into the IC. Therefore, in order to inhibit rapid turn-on and turn-off of the IC to be protected, it would be advantageous to provide hysteresis to the protection circuit. The hysteresis would allow the temperature to fall by a few degrees before allowing the IC to power-up.
Accordingly, it is an object of the present invention to provide a new and more effective thermal protection circuit that will effectively and efficiently sense the temperature of a solid state subscriber line interface circuit and shut-down the circuit when a designated upper limit temperature is reached.